Well known are technique in which the sensitive wavelength range of a silver halide emulsion is expanded by adding thereto a sensitizing dye to optically sensitize it.
Many known spectral sensitizing dyes are used for this purpose. They include, for example, a cyanine dye, a merocyanine dye and a xanthene dye each described at pages 198 to 228 of The Theory of the Photographic Process, the third edition, edited by T. H. James, 1966, published by Macmillan Co., Ltd.
When these sensitizing dyes are applied to a silver halide emulsion, not only do they expand the sensitive wavelength range of the silver halide emulsion, they must also satisfy the following conditions:
(1) spectral sensitizing range is appropriate, PA0 (2) sensitizing efficiency is good and a sufficiently high sensitivity can be obtained, PA0 (3) fog does not result, PA0 (4) fluctuation in sensitivity due to a fluctuation in temperature during exposure is small, PA0 (5) there are no bad interactions with other additives, for example, a stabilizer, an anti-foggant, a coating aid and a color developer, PA0 (6) sensitivity does not fluctuate when a silver halide emulsion containing a sensitizing dye is stored, even at a high temperature and humidity, and PA0 (7) an added sensitizing dye, which is diffused into the other light-sensitive layers, does not cause a color stain (a color mixing) after development.
The above conditions are important during the preparation of a silver halide emulsion used in a silver halide photographic light-sensitive material. In spite of various attempts, however, reduction in sensitivity of an unprocessed sample during storage has not been prevented to a satisfactory degree.